The present invention relates to a fuel cell system adapted to start up a fuel cell stack, mainly using an electric power charged in a secondary battery, which is installed on a fuel cell powered vehicle whereby, through the use of an electric power output generated by the fuel cell stack and the electric power charged in the secondary battery, auxiliary units associated with the fuel cell stack for electric power generation are driven while producing a vehicle drive torque output.
An attempt has been made to start up a fuel cell system through the use of a technology wherein an electric power of a secondary battery other than a fuel cell stack is applied to an air compressor to allow oxidant gas to be supplied to the fuel cell stack to start up the same to be available for electric power generation whereupon a shift occurs from the secondary battery to the fuel cell stack made available for electric power generation to utilize the electric power output generated thereby as a driving power supply, as disclosed in Japanese Patent Application Laid-Open No. 2001-229943.
With such a fuel cell system, during a mode in which the electric power output of the secondary battery is supplied to the air compressor as the driving power supply, an output voltage and an electric power output are arbitrarily controlled by a DC/DC converter. The DC/DC converter is configured such that, during start-up operation, the electric power available from the secondary battery is made to be sufficiently higher than the electric power generated by the fuel cell stack that is adequately warmed up to be available for obtaining a desired electric power output. And, an output voltage of the electric power generated by the fuel cell stack is clamped at an output voltage level of the DC/DC converter, thereby precluding the electric power of the fuel cell stack from exceeding a certain desired level.
With such a fuel cell system, when discrimination is made the fuel cell stack is available for electric power generation, by decreasing the output voltage of the DC/DC converter while increasing the electric power to be supplied from the fuel cell stack, a source of electric power supply to the air compressor is made available to be smoothly shifted from the secondary battery to the fuel cell stack.